Coil antennas mounted in mobile electronic devices used in RFID systems are disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2002-325013 (Patent Document 1), Japanese Unexamined Patent Application Publication No. 2002-373319 (Patent Document 2), and Japanese Unexamined Patent Application Publication No. 2005-94319 (Patent Document 3).
FIG. 1 is a top view illustrating the structure of an antenna coil described in Patent Document 1. An antenna coil 30 illustrated in FIG. 1 includes an air core coil 32 and a planar magnetic core member 33. The air core coil 32 is configured by spirally winding conductors 31 (31a, 31b, 31e, and 31d) in a plane on a film 32a. The magnetic core member 33 is inserted into the air core coil 32 so as to be substantially parallel to a plane of the air core coil 32. The air core coil 32 has an aperture 32d and the magnetic core member 33 is inserted into the aperture 32d. A first terminal 31a is connected to a connecting conductor 31e via a through hole 32b, and a second terminal 31b is connected to the connecting conductor 31e via a through hole 32c. The magnetic-material antenna is arranged on a conductive plate 34.
In an antenna coil disclosed in Patent Document 2, an antenna magnetic core, which is a planar body, is arranged so as to configure substantially the same plane as the antenna coil while passing through an air core unit of the antenna coil.
In a coil antenna disclosed in Patent Document 3, multiple coils wound on a plane are arranged in multiple layers around the same central axis, the coils on the respective layers are connected in series to each other, and a member of a high permeability is provided between the coils on the respective layers.
In general, the characteristics as a coil antenna are improved and its communication performance is also improved as the number of turns of the coil is increased, the loss of the coil is decreased, and the width of the magnetic core is increased, as long as an inductance necessary for the resonation at a predetermined resonant frequency is achieved. However, there are the following problems in the coil antennas disclosed in Patent Documents 1 to 3.
The antenna coils disclosed in Patent Documents 1 and 2 have the problems in that it is necessary to decrease the width of the magnetic-material core in order to increase the number of turns of the coil, it is not possible to increase the number of turns of the coil when the width of the magnetic-material core is increased, and the loss of the coil is increased when the line width of the coil conductor is decreased to increase the number of turns.
In addition, the antenna coil disclosed in Patent Document 1 has a structure in which the antenna coil is coupled to the magnetic flux parallel to the rear conductive plate 34, as illustrated in FIG. 1. Accordingly, when the antenna coil is mounted in, for example, a mobile phone terminal, there is a problem in that the mobile phone terminal cannot be used with being held over the surface of a reader-writer in parallel if the antenna coil is installed in parallel with a circuit board in the casing of the mobile phone terminal.
In the coil antenna disclosed in Patent Document 3, since the member of a high permeability (a magnetic-material core) is vertically directed, the communication is disabled if the coil antenna is placed on a conductor plate.